Prion protein (PrP) is a glycophosphatidylinositol (GPI)-linked glycoprotein found on the surface of a wide variety of cells, including neurons and immune cells. A pathogenic form of PrP causes spongiform encephalopathies such as Creutzfeldt-Jakob and Mad Cow diseases, which are fatal and for which no cure exists. Cellular PrP is highly conserved among mammals, suggesting an important role in cellular function. However, the normal function of the protein remains enigmatic. Prnp-/- (PrP-deficient) mice are slightly less susceptible than wild-type mice to infection with wild-type herpes simplex virus 1 (HSV-1), suggesting that PrP interacts either with a cellular pathway utilized by the virus or with a host antiviral defense mechanism. HSV effectively counteracts many cellular antiviral responses;therefore, if PrP normally functions in antiviral defense, a wild-type virus may neutralize this defense to maintain replication and virulence and so would have relatively little phenotype in PrP-deficient mice. An attenuated HSV mutant that cannot counteract cellular antiviral responses, however, may reveal a defect in the response to HSV if it regains its capacity to replicate and spread in PrP-deficient mice. We have observed that PrP-deficient mice are more susceptible to infection with a HSV-1 ICP34.5 mutant virus, or an ICP34.5 mutant unable to bind and sequester the autophagy protein beclin-1. We have also observed that titers of the ICP34.5 beclin binding mutant recover to wild-type virus levels specifically in the brainstems of PrP-deficient mice, suggesting that PrP plays an antiviral role in this tissue. ICP34.5 counteracts the phosphorylation of eIF21 to maintain viral protein synthesis and inhibits autophagy to reduce virus particle elimination in infected cells. Therefore, we hypothesize that cellular PrP functions in either the PKR signaling or autophagy pathways. We will utilize HSV mutant viruses to elucidate the linkage between PrP and specific antiviral pathways in vivo and in vitro. In Aim 1, we will determine the extent of the defect in antiviral responses in PrP-deficient mice and whether it is specific to interactions involving the HSV-1 ICP34.5 protein. In Aim 2, we will determine whether PrP functions in autophagy in vivo. Finally, to further elucidate PrP function, in Aim 3 we will test the hypothesis that a primary cell culture system can be established which replicates in vitro the defect in PrP-deficient mice, and use it to determine where the PrP-related defect exists in cell signaling or induction of autophagy. Our overall goal is to use these unique virus tools to elucidate an antiviral function of normal cellular PrP which may provide insight into the neurodegeneration accompanying conversion of PrP to its pathogenic form. PUBLIC HEALTH RELEVANCE: Prion protein (PrP) is a highly conserved and widely expressed protein whose pathogenic form causes fatal spongiform encephalopathies. We have observed that PrP-deficient mice are more susceptible to infection with herpes simplex virus 1 (HSV-1) mutants of ICP34.5 whose interactions with host proteins are well- characterized. Our goal is to use these unique virus tools to elucidate an antiviral function of normal cellular PrP and thus gain insight into neurodegeneration accompanying conversion of PrP to its pathogenic form.